1. Field of the Invention
The present invention relates to a positively chargeable toner for two-component development which is used for development of an electrostatic latent image formed in electrophotography, electrostatic recording method, electrostatic printing method, or the like, a positively chargeable two-component developer comprising the positively chargeable toner, and a process for preparing the positively chargeable toner.
2. Discussion of the Related Art
In recent years, with the development of on-demand printing, there has been required a positively chargeable two-component developer giving high-quality image and high speed. Especially, in order to meet the requirement of high-quality image, a ferrite carrier having a low saturation magnetization as compared to that of iron powder or magnetite has been used (Japanese Patent Laid-Open Nos. Sho 59-104663, 2000-330342, Hei 10-104884 and the like). However, when the saturation magnetization is lowered, magnetic brush is weakened, so that an ability of scraping off a toner adhered to a photoconductor is weakened, thereby causing filming.
Especially, when an external additive such as silica or titanium oxide is added from the viewpoints of securing chargeability and fluidity, the external additive is freed when stirring during triboelectric charging, or an external additive originally freed is adhered to a photoconductor, so that the external additive acts as a core to form a coating film for a toner, thereby causing filming of a toner on the photoconductor.
Japanese Examined Patent Publication No. Sho 63-55701 discloses a toner having a small change in fluidity even after running by mixing a toner in which a silica is embedded in a toner surface, with a silica not embedded in a toner surface. However, in a long-term and high-speed printing, a stress against a toner or a photoconductor is increased, so that the silica on a photoconductor acts as a core, whereby filming is likely to be caused. Especially, in the positively chargeable two-component developer, this tendency is remarkable, because a positively chargeable silica is likely to be transferred to a carrier.
Therefore, there have been proposed to reduce filming by completely removing free, fine inorganic particles which are not adhered to a toner in Japanese Patent Laid-Open No. Sho 63-139366, or by using titanium oxide having a specified value in the correlation coefficient of an approximation curve showing the adhesion state in Japanese Patent Laid-Open No. 2000-267357. However, in the former method, when printing is run for a long term, the fluidity of the toner is lowered because the fine inorganic particles adhered to the toner are embedded in the toner, so that there arise some problems in printed images such as decrease in the image density and background fogging. In the latter method, since titanium oxide is very hard, there is a risk of damaging a photoconductor.
An object of the present invention is to provide a positively chargeable toner for two-component development for stably giving a high-quality fixed image without causing photoconductor contamination even in high-speed copy machines or printers; a positively chargeable two-component developer comprising the positively chargeable toner; and a process for preparing the positively chargeable toner.
These and other objects of the present invention will be apparent from the following description.
According to the present invention, there are provided:
(1) a positively chargeable toner for two-component development comprising:
a resin binder;
a colorant;
a releasing agent comprising a wax having a melting point of 50xc2x0 to 120xc2x0 C.; and
an external additive comprising a positively chargeable silica, the positively chargeable silica having an absolute deviation of an error of 0.1 or less, against an approximate straight line showing an adhesion state of silicon atoms to carbon atoms, and a free ratio of 5% or less, wherein the positively chargeable toner is usable together with a ferrite carrier having a saturation magnetization of from 40 to 100 Am2/kg;
(2) a positively chargeable two-component developer, comprising the toner of item (1) above, and a ferrite carrier having a saturation magnetization of from 40 to 100 Am2/kg; and
(3) a process for preparing a positively chargeable toner for two-component development, comprising the steps of mixing an untreated toner comprising a resin binder, a colorant, and a releasing agent comprising a wax having a melting point of 50xc2x0 to 120xc2x0 C., with an external additive comprising a positively chargeable silica, or a mixture of a positively chargeable silica and a negatively chargeable silica, thereby surface-treating the untreated toner with the external additive; and thereafter sieving the resulting toner, wherein an entire silica in the resulting toner has an absolute deviation of an error of 0.1 or less, against an approximate straight line showing an adhesion state of silicon atoms to carbon atoms, and a free ratio of 5% or less.